This invention aids in the remediation of environmental contaminants in subsurface soils and groundwater via the stimulation of anaerobic processes. Specifically, this invention relates to remediation processes involving the emplacement of solid-phase or aqueous-phase treatment agents. Emplacement of dried seaweed or kelp species as electron donors for microorganisms that carry out reductive dechlorination of chlorinated solvent source areas or plumes is illustrative of the invention.
Various species of Seaweed including Ascophyllum nodosum, Dulse, Nori, and Kelp contain substantial nutrients, beneficial to anaerobic processes. Seaweeds are available in a variety of forms including sheets, meals, flakes and powders that can either be hydrolyzed for solubility or remain insoluble as a slow release remedial product. In addition, dried seaweed can come in various sizes ranging from large granules characteristic of insoluble kelp meal to high mesh sizes of fine powder. The dynamic nature of seaweed has resulted in its wide use in varying commercial fields. Liquid seaweed extract as well as insoluble and (hydrolyzed) soluble dried seaweeds are commonly used as fertilizers to enhance the development and growth rate of plants. Seaweed is also used as a food additive for livestock to promote growth and health. Furthermore, seaweed has been an essential food source for years, used in sushi, chips, seasoning, and even as a dietary supplement for its high nutritional value.
The chemical composition of seaweeds allows for the contribution of stimulating is comprised of fatty acids, carbohydrates, and proteins. Their concentrations of vitamins B2, and B12 in particular make seaweed an excellent alternative for environmental remediation. The species Ascophyllum nodosum contains high levels of enzymes, 17 amino acids, macro and micronutrients, plant hormones (auxins, cytokins, gibberillins), 25% alginic acid, and over 50% of carbohydrates and polysaccharides. Seaweeds also have over 72 minerals, and assorted vitamins (B2, B12, K) that encourage the vigorous and healthy growth of subsurface biological life. Seaweed and Kelp offer the necessary micronutrients and volatile fatty acid precursors that will provide long-term production of organic hydrogen necessary for reductive dechlorination of chlorinated solvents in groundwater and soils. The high concentrations of many of these valuable nutrients provide optimal living conditions for the anaerobic processes responsible for the remediation of contaminated soil and groundwater sites.
Chlorinated solvents are the most common class of ground water contaminants at hazardous waste sites in the U.S. In a list of the top 25 most frequently detected contaminants at such sites, the Agency for Toxic Substances and Disease Registry (ATSDR) found that 10 of the top 20, including two of the top three, were chlorinated solvents or their degradation products. National Research Council, Alternatives for Ground Water Cleanup (National Academy Press, Washington, D.C. 1994). In fact, the same survey found that the most common contaminant, trichloroethylene (TCE), is present in more than 40% of National Priority List sites. The remediation of ground water contaminated by these compounds often presents unique obstacles related to their inherent characteristics, including hydrophobicity and high density. Many commercial process utilize raw vegetable oils and emulsions which co-elute the targeted solvents within the treatment liquid masking the presence of the compound targeted for treatment rather than stimulating the mineralization of said compound.
Natural attenuation of chlorinated solvents by reductive dechlorination often occurs at sites where an electron donor (food source or substrate for microbes) is present along with the chlorinated solvent contamination. As dissolved oxygen and other electron acceptors become depleted some microbes are capable of using the chlorinated solvents as electron acceptors. For selected compounds such as chlorinated ethylenes sequential dechlorination to a harmless byproduct ethylene can be achieved under favorable environmental conditions (EPA/600/R-10 98/128 September 1998).
In recent years efforts have been made to produce this anaerobic treatment effect by injection of electron donor into the subsurface. An overview of these technologies can be reviewed in the EPA document Engineered Approaches to In Situ Bioremediation of Chlorinated Solvents: Fundamentals and Field Applications (EPA 542-R-00-008 July 2000). Other inorganic and organic compounds can be degraded or immobilized under anaerobic conditions including selected toxic metals, nitrate, and MTBE. For sites that do not have sufficient amounts of natural electron donors to drive anaerobic natural attenuation, injection of microbial substrates has proven to be a cost-effective treatment or plume migration control measure. The microbial substrates can be injected into the contaminant source area where residual contamination is adsorbed onto soils or injected in a line across a ground water contaminant plume to form a permeable reactive wall to prevent further contaminant migration.
A wide variety of sugars, alcohols, organic acids, and even molecular hydrogen have been used successfully as electron donors to enhance anaerobic biotransformation processes. Most of these compounds are rapidly consumed after injection and must be replaced by either continuous low concentration delivery systems or with frequent batch additions of additive solution. Contaminant source areas can not be effectively removed or even precisely located for many ground water contaminant plumes. The presence of residual chlorinated solvents adsorbed onto soils or present as dense non-aqueous phase product (DNAPL) serves as an example of persistent ground water plume source areas that can last for many decades. These persistent contaminant source areas continue to contaminate ground water for many years such that continuous operation of recirculation systems or frequent substrate injections can be very costly over the life of a project. Long-term injection of substrates into wells or infiltration galleries often leads to severe bacterial fouling problems adding to project operation and maintenance costs.
Recent interest has developed in the use of materials that slowly biodegrade or slowly release organic matter into ground water over time. A variety of vegetable oils have been demonstrated to be effective electron donors to stimulate anaerobic biodegradation. Although edible oils such as soybean oil have a much lower viscosity than a semisolid product, distribution in saturated soils is difficult. Soybean oil has a viscosity approximately times higher than water, which results in multiphase fluid flow and potential oil blockage of soil porosity. Injection of pure oil or large droplets of emulsified oil blocks soil pores producing treatment zones that are ineffective because they prevent free flow of ground water through the oil treated area. Injection of pure soybean oil into porous soil media has been shown to reduce water permeability by up to 100%.
In addition to slowly biodegradable hydrogen sources, soil and groundwater remediation process that utilize zero-valent metals have been applied with varying success. In the second embodiment of the invention, the addition of zero-valent metals to the micro dried seaweed or kelp allows for maintained reducing conditions resulting in greater longevity of the reactive metal surface. Zero-valent metal particles have been proven to effectively degrade halogenated solvents. For example, the mechanism and reaction rates of which iron reduces chlorinated aliphatics has been studied extensively due to iron's low cost and low toxicity. Additionally, the pathways of the dehalogenation of DNAPL's such as TCE have been proposed. TCE undergoes hydrogenolysis where the replacement of each of the three chlorines occurs sequentially. TCE reduces to cis-1,2-dichloroethene, trans-1,2-dichloroethene, and 1,1-dichloroethene. These intermediates in turn reduce to ethene and ethane.